Nonvolatile memory devices retain their stored data even when their power supplies are turned off. Thus, the nonvolatile memory devices are widely used in computers, mobile telecommunication systems, memory cards and so on.
Recently, other types of nonvolatile memory devices, for example, phase change memory devices are being used. A unit cell of a phase change memory device includes a switching device and a data storage element serially connected to the switching device. The data storage element has top and bottom electrodes and a phase change material layer interposed therebetween, and the bottom electrode is electrically connected to the switching device.
In general, the bottom electrode acts as a heater. When a write current flows through the switching device and the bottom electrode, Joule heat is generated at an interface between the phase change material layer and the bottom electrode. Such Joule heat converts the phase change material layer to an amorphous state (reset state) or a crystalline state (set state). The phase change material layer having the amorphous state exhibits a higher resistance than the phase change material layer having the crystalline state. Accordingly, the phase change material layer is widely employed as the data storage element of the phase change memory device.
The switching device should be designed to have a current drivability sufficient to supply the write current. However, an area occupied by the switching device should be increased in order to enhance the current drivability. When the area of the switching device is increased, it is difficult to improve the integration density of the phase change memory device.
An alloy layer of germanium (Ge), antimony (Sb) and tellurium (Te) (hereinafter, referred to as a “GeSbTe” layer) is widely used as a phase change material layer. A set pulse signal having a long time of several hundreds of nano seconds is required to crystallize the GeSbTe layer. Accordingly, there may be a limitation in improving the write speed (program speed) of the phase change memory device employing the GeSbTe layer. Also, a reset pulse signal having a high write current (reset current) of about 0.8 mA to about 1 mA is required to transform the GeSbTe layer into the amorphous state (reset state). Accordingly, when the GeSbTe layer is employed as the phase change material layer of the phase change memory device, there may be a limitation in reducing power consumption in a write (program) mode of the phase change memory device.
The GeSbTe layer may be doped with impurities such as silicon atoms or nitrogen atoms. In this case, the GeSbTe layer may have small and uniform grains due to the impurities. Accordingly, energy required to transform the GeSbTe layer into an amorphous state may be reduced to further decrease the reset current of the GeSbTe layer. However, the impurities within the GeSbTe layer disturb the GeSbTe layer from being crystallized. Consequently, when the GeSbTe layer is doped with the impurities, the reset current of the doped GeSbTe layer is decreased whereas the set pulse width of the GeSbTe layer is increased.
An optical information recording medium employing GeBiTe layer as a phase change material layer is disclosed in U.S. patent publication No. 2005/0227035 A1 to Fuchioka et al., entitled “Information Recording Medium”.
Another information recording medium employing GeBiTe layer as a phase change material layer is disclosed in U.S. Pat. No. 6,858,277 B1 to Yamada et al., entitled “Information Recording Medium and Method for Manufacturing the Same”.